The goal of the proposed research is to identify by mutation indispensable genes in the small soil nematode Caenorhabditis elegans and to investigate microscopically the developmental consequences of mutation in such genes. C. elegans has been chosen for this work because of its suitability for genetical methods and its relative cellular simplicity. The specific objectives of the proposal are: First, to isolate genetically marked chromosome rearrangements that can be used as balancers to maintain recessive lethal and sterile mutations in heterozygotes and to facilitate their subsequent manipulation; second, to saturate some regions of the genome, covered by balancers, with recessive lethal and sterile mutations induced by ethyl methansulfonate and to assign the mutations to genes by complementation; and third, to categorize any detectable defects in cell lineage in the mutants by light microscopy.